Thought I'd mention I'm starting a mini-project to make map feature overlays for planets and satellites. These are set up on a cylindrical projection so they can be displayed with Science On A Sphere, Celestia, and the like. The key is that text for feature names has to be "pre-distorted" so it looks correct when viewed on a sphere. This is especially important at high latitudes. The overlays are for now in the form of transparent PNG images.

So far I've started with Mars and Enceladus. The programming language is IDL, and the lat/lon info for the feature names is from the USGS Map-A-Planet Gazeteer. I'll try to post some results once they are a little better.

Enceladus is a special case, since the map I had put together is in planetocentric coordinates, and the feature lat/lons are in planetographic. So a conversion of one or the other would need to be done.

I have now a preliminary map of Mars with the features overlayed onto the texture image. A threshold size of 350km for feature diameter is being applied. A check is made to suppress multiple features that would print on top of one another. Priority is given to "Mons" features and large features.

"The PDS recommends that the planetographic longitude standard be followed, but also supports the traditional method. Specifically, the longitude range of (-180, 180) is supported for the Earth, Moon and Sun"

John - I had used the 180 center longitude that currently appears in the Science On A Sphere website for Mars, and as Doug mentions a lot of planetary maps are done with the 180 degree center convention. Do I recall that Celestia works better longitude zero in the center?

Accordingly I made some new ones in my web directory with longitude zero in the center.

The funny thing is that the quasi-standard way of doing things with SOS is actually longitude 0 in the center. Meanwhile most of the planetary maps on my own site have longitude 180 in the center. So as long as we know what it is we should be OK, particularly if we want to display with Celestia.

Meanwhile, here's the latest with Enceladus. The tiger stripes may need some work in terms of rotating the labels.

Also looks like the labels are generally shifted a few degrees so the west, so possibly the zero longitude reference system is different between the USGS Gazeteer and my map. This should be possible to adjust.

Don't get too hung up on the question of having 0 or 180 in the middle. There is no right or wrong. Sometimes you need to fit a standard required by software, maybe, but think of Earth. Put 0 in the middle, 180 at the edges, and you are splitting the Pacific Ocean in two. If you want to show the Pacific whole, put 180 in the middle. Every application has its own needs. Just do what works.

Doug's PDS quote has nothing to do with the central longitude of a map. It is just saying you can use negative longitudes (up to -180) in those specified cases, but elsewhere longitudes are defined just as 0 to 360.

The longitude system on Enceladus and the other icy satellites was shifted around 2008- in the case of Enceladus, by about 3.5 degrees. This was because the longitude systems on the satellites are defined relative to geographical features, not relative to the Saturn-facing direction (though the definitions are designed so that zero longitude points approximately at Saturn). By 2008, Cassini mapping of the satellites was complete enough to better determine the locations of the craters used to define the longitude systems, which required shifting the longitude system so that those craters had the "correct" longitudes.

Perhaps this is the reason for the discrepancies in those label positions?

Someone should make a map with 0 Longitude going down the center instead of the equator. At least then we would have correct perspective on the tiger stripes.

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Space Enthusiast Richard Hendricks --"The engineers, as usual, made a tremendous fuss. Again as usual, they did the job in half the time they had dismissed as being absolutely impossible." --Rescue Party, Arthur C ClarkeMother Nature is the final inspector of all quality.

Hendric, yes that would be an interesting presentation, basically rotating the data 90 degrees around the X or Y axis when making the cylindrical projection. The labels should look OK if the map in post #10 is so transformed.

Meanwhile, the general intent with these feature maps is to view them with a program such as Celestia, so one can see the features and labels with a proper perspective hovering over the location of choice. In the case of Celestia the tiger stripes and labels have the correct perspective, though the well-known Celestia polar pinch effect shows up here.

The longitude system on Enceladus and the other icy satellites was shifted around 2008- in the case of Enceladus, by about 3.5 degrees. This was because the longitude systems on the satellites are defined relative to geographical features, not relative to the Saturn-facing direction (though the definitions are designed so that zero longitude points approximately at Saturn). By 2008, Cassini mapping of the satellites was complete enough to better determine the locations of the craters used to define the longitude systems, which required shifting the longitude system so that those craters had the "correct" longitudes.

Perhaps this is the reason for the discrepancies in those label positions?

John

Indeed John, the Enceladus labels in post #10 have a 3.5 degree shift compared with post #8 and that appears to make things somewhat better. I had thought I shifted the underlying map to account for this and I'm guessing the USGS Gazeteer might have the older coordinate system? We might check this by looking at the particular crater(s) that is the reference point.

There are a couple of other considerations as well. First is that I wrote a routine to convert the USGS label data in planetographic coordinates to my map that is in planetocentric. Hopefully that is working correctly. Still there might be on the order of a degree or two of error in the construction of the map. If there remains a source of systematic longitude shift this presents an opportunity to try and correct things.

That is called a Transverse Cylindrical Projection, perhaps most familiar to us in the expression UTM (Universal Transverse Mercator), a coordinate system which maps long narrow strips (gores) 6 degrees wide and 160 degrees long, from 80 north to 80 south, in 60 zones around the planet. Within each zone, you are looking at a cylindrical projection running along the meridian.

So this can be done, and any map reprojection software which allows map data to be output in a transverse cylindrical projection would do it for you.

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